Rubber mountings of this kind are used for mounting the drive units in vehicles of all types. In regard to mounting internal combustion engines in motor vehicles, the applicable considerations are to some extent in conflict. On the one hand, in order to avoid the transmission of noise, it is desirable or necessary to provide the softest possible mounting with a low level of natural damping, which however allows the movements of the engine, which are generated, for instance, by movement over the road surface, to become very great and almost go into a condition of free oscillation. On the other hand, the substantial movements of the engine can be reduced by using hard engine mountings or separate oscillation dampers, which, however, in turn results in a considerable amount of noise being transmitted to the body.
Rubber mountings of this general kind are known (for example, published European application No. 27,751 and published European application No. 12,638), which provide for generally good damping in the low-frequency range. The disadvantage however is that cavitation occurs in the chamber or chambers above a given amplitude of oscillation or a given frequency, and such cavitation results in the production of undesirable noise. Cavitation of that nature occurs when, in the event of substantial deflection movements, the damping fluid flows from one chamber into the other and a vacuum is formed on one side of the throttle means, as the fluid compensation effect cannot take place sufficiently quickly.
Cavitation of a liquid results when the pressure in a point in the liquid decreases to a value which is below the vapor pressure of the fluid. When the vapor pressure of a fluid is greater than the pressure at a point therein, the fluid changes from the fluid phase, that is, liquid phase, into the vapor phase. This change of phase is akin to boiling. The vapor will be produced during cavitation as long as the pressure within the fluid is below that of its vapor pressure. Of course, the vapor pressure of fluids within hydraulic damping equipment causes cavitation at pressures well below atmospheric pressure, that is, the fluids boil at operating temperatures.
In the newer and smaller motors, which are generally of four-cylinder design, the vibrations in the motor are at substantially half the frequency of the vibrations of an eight-cylinder motor running at the same number of revolutions per minute. Therefore, the frequency of excitation of the motor is at generally lower frequencies than in corresponding six- and eight-cylinder motors. Furthermore, low frequency vibrations especially during start-up and shut-down of the engine are more prevalent because of the inherent nature of the lower frequencies of vibration due to the smaller number of piston explosions, etc. Additionally, the relative weight of the pistons to the entire engine in a four-cylinder motor are generally greater than in a larger motor. A four-cylinder motor generates greater amplitudes of vibration. Since these greater amplitudes of vibration are at a lower frequency, as is well known in the vibration theory, these lower frequency vibrations will have greater amplitudes than similar higher frequency vibrations of similar energy, because the low pass characteristics of a smaller engine are not as highly filtered as those of a larger engine. Therefore, in smaller engines which are lighter in weight and do not damp out the frequencies of oscillation the same way that larger motors with a greater number of cylinders do, the danger of cavitation in the engine mounts is greater. Because of the problems which have developed in the trend towards lighter, smaller cars with correspondingly lighter, smaller motors with less cylinders, there has been a growing need for refinement of the engine mounts.
With the foregoing in mind, an object of the present invention is to provide a simple and effective rubber mounting which avoids cavitation phenomena, involving interferring noise, in the low-frequency range at large amplitudes caused by excitation of the engine due to the road surface or in the starting-up or stopping phase, and wherein optimum insulation in respect of the transmission of noise from the engine to the body work in the high-frequency range, at low amplitudes, is not detrimentally affected.
To achieve that object, the present invention generally provides that the partitioning wall is provided, in the region of its periphery, and on the side which is toward the rubber-elastic spring element, with a spring which is disposed in a prestressed condition between the peripheral wall and a support means extending in the region of the outer periphery of the petitioning wall, and that arranged on the side of the partitioning wall which is remote from the rubber-elastic spring element is a sealing means which extends over the periphery of the arrangement and which cooperates with a sealing surface of one of the end walls and which seals the compensation chamber relative to an annular space which is disposed radially between the partitioning wall and the peripheral wall of the pressure chamber, wherein the spring or the support means has recesses which form a constant communication between the pressure chamber and the annular space. The pressure chamber is, of course, the chamber which includes the rubber-elastic peripheral wall portion, and the compensation chamber is the chamber disposed on the other side of the partitioning wall, and typically constructed with a bellows or concertina-type membrane.
An advantage in an above described arrangement is that, in the normal range of operation of the mounting, the functions of noise insulation and damping are not affected, whereas at large amplitudes at which cavitation occurs, there is a fluid exchange effect, with a bypass action relative to the damping passage. Such situations often occur in the phase of starting up and stopping an engine, and when the engine is subjected to substantial excitation by virtue of the road surface. The reduced pressure which is produced in the chamber under cavitation conditions acts in opposition to the prestressing force of the spring so that the partitioning wall, with its sealing means, moves away from the sealing surface and permits an exchange of fluid past the sealing means, through the annular space and the recesses. The axial distance covered by the partitioning wall not only permits a fluid interchange effect, but in addition the volumes of the two chambers also change as a result thereof. An advantage in that connection is that the volume of the chamber in which there is a reduced pressure is decreased so that the action also acts in opposition to the cavitation phenomena. When the movement is reversed, the spring is released of the added stress, and the bypass closes.
An advantageous embodiment provides that the partitioning wall is arranged radially within a support ring, and that the spring which comprises rubber or another elastomer is vulcanized to the support ring.
Another feature provides that the spring for the partitioning wall is of L-shaped or T-shaped cross section, and the recesses or openings are disposed in the limb portion thereof, which is in a prestressed condition and which bears against the support means. The openings or recesses are so selected that, when the bypass opens, sufficient fluid can flow through the recesses by way of the annular space.
In accordance with a particularly advantageous construction, the sealing means is of substantially square or rectangular cross section and is fixedly connected to the previously mentioned support ring. For purposes of satisfactory functioning, the sealing means is associated with and fixedly connected to the partitioning wall. However, it would also be possible for the sealing means to be a component on the opposite side, and for the part of the partitioning wall to be formed as a sealing seat.
Another feature provides that the sealing means has at least one projection which extends in the direction of the sealing surface and which is formed as a sealing lip which extends over the entire periphery. In such an arrangement, the sealing lips may advantageously and easily be in the form of, for example, half-round projections so as to give line contact at the sealing seat, such line contact ensuring a satisfactory sealing action so that, due to the prestressed condition obtaining in the normal range of operation, there cannot be any interchange of fluid past the sealing means and through the annular space, which could adversely affect the damping qualities of the rubber mounting.
In order to ensure ease of production and manufacture of the system, a particularly desirable construction provides that a rigid annular member which is fixed in the clamping region is provided as the support means for the partitioning wall spring and/or as the sealing surface. With such a construction, it is readily possible for the annular members, together with the partitioning wall, to be formed as a separate component, and fitted into the mounting as a unit.
Another embodiment provides that the support means is a component of the connecting flange or the annular member. In that arrangement, the spring can be supported directly, without additional members.